Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a tracer gas which passes through the smallest of leaks in a sealed test piece. After passing through a leak, a test sample containing helium is drawn into a leak detection instrument and measured. An important component of the instrument is a mass spectrometer tube which detects and measures the helium. The input test sample is ionized and mass analyzed by the spectrometer tube in order to separate the helium component. In one approach, the test piece is pressurized with helium. A sniffer probe connected to the test port of the leak detector is moved around the exterior of the test piece. Helium passes through leaks in the test piece, is drawn into the probe and is measured by the leak detector. In the most commonly-used approach, the interior of the test piece is coupled to the test port of the leak detector and is evacuated. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector.
One of the requirements of the spectrometer tube is that the inlet through which the helium is received be maintained at a relatively low pressure, typically 2.times.10.sup.-4 torr. In a so-called conventional leak detector, a diffusion pump and associated forepump and a cold trap are utilized to maintain the input of the spectrometer tube at the required pressure. The conventional leak detector provides satisfactory performance under a variety of conditions, but has certain drawbacks. The test port, which is connected to the test unit or to the sniffer probe, must be maintained at a relatively low pressure. Thus, the vacuum pumping cycle is relatively long. Furthermore, in the testing of leaky or large volume parts, it may be difficult or impossible to reach the required pressure level. If the required pressure level can be reached, the pumping cycle is lengthy.
Many of these problems were eliminated in the counterflow leak detector disclosed by Briggs in U.S. Pat. No. 3,690,151. A counterflow leak detector is also disclosed by Fruzzetti et al in U.S. Pat. No. 4,499,752. In the counterflow leak detector, the mass spectrometer tube is connected to the inlet of a diffusion pump, and the helium tracer gas is introduced through the foreline, or normal output port, of the diffusion pump. The diffusion pump exhibits a high pressure ratio for heavier gases, but a low pressure ratio for lighter gases such as helium. Therefore, helium diffuses at an acceptable rate in a reverse direction through the diffusion pump to the spectrometer tube and is measured. Heavier gases in the sample are to a large degree blocked by the diffusion pump and are prevented from reaching the spectrometer tube.
The technique of reverse flow of helium through the diffusion pump permits the leak detector test port to be operated at the pressure required at the diffusion pump foreline. This pressure is several orders of magnitude higher than the required operating pressure of the spectrometer tube. Performing leak tests at a higher test pressure is advantageous for several reasons. Leaky or large volume pieces can be tested, since the test pressure is relatively easy to attain. In addition, vacuum pumping of the test piece, regardless of its size or leak rate, is faster. The counterflow leak detector has provided highly satisfactory operation under a wide variety of conditions.
Commercially available counterflow leak detectors typically can measure a range of leaks from 1.times.10.sup.-9 std cc/sec to 1.times.10.sup.-5 std cc/sec in four ranges: 0-1.times.10.sup.-8, 0-1.times.10.sup.-7, 0-1.times.10.sup.-6 and 0-1.times.10.sup.-5 std cc/sec. Should the leak exceed 1.times.10.sup.-5 std cc/sec, the meter on the leak detector goes off-scale, and the operator must remove the probe and wait for the helium in the system to be pumped out. The four decade range of prior art counterflow leak detectors is established primarily by the input range of the mass spectrometer tube. While the above range of leak detector operation is usually satisfactory, there are some cases where more than four decades of operation are required. Techniques which involve modifications to the spectrometer tube and additional electronic circuitry have been developed for increasing the operating range of leak detectors. This approach has proven to be expensive and complex.
The maximum allowable test port pressure is higher for the counterflow configuration than for conventional leak detectors. Nonetheless, reaching even the higher test port pressure can be difficult when testing large volumes, dirty parts, or parts with large leaks.
It is a general object of the present invention to provide an improved helium mass spectrometer leak detector.
It is another object of the present invention to provide a leak detector capable of measuring a wide range of leak rates.
It is a further object of the present invention to provide a leak detector which can be operated at a relatively high test port pressure.
It is yet another object of the present invention to provide a leak detector having a diffusion pump with a low sensitivity foreline and a high sensitivity foreline.
It is still another object of the invention to provide a leak detector having high and low sensitivity operating modes.